1. Field of the Invention
The present invention relates to a liquid crystal display element and a liquid crystal display device employing the same, e.g., a liquid crystal projector.
2. Description of the Related Art
The liquid crystal display modes for the reflection type liquid crystal light valve employing a nematic liquid crystal are roughly classified into the three kinds of systems. That is, there are a tilted homeotropic ECB (Electrically Controlled Birefringence) mode, a homogenious ECB mode and a reflection type twisted nematic mode.
By the tilted homeotropic ECB mode is meant the alignment system in which the liquid crystal molecules are arranged in such a way as to be slightly inclined with respect to the perpendicular direction of each of a pair of substrates by utilizing the oblique evaporation method or the like, and this mode is described in an article of Applied Physics Letters 20, 199 (1972) for example.
By the homogenious ECB mode is meant the alignment system in which the liquid crystal molecules are arranged in such a way as to be roughly parallel to each of a pair of substrates, and this mode is described in JP-A-1-7021.
Also, by the reflection type twisted nematic mode is meant the system in which the liquid crystal molecules are arranged in such a way as to be roughly parallel to each of a pair of substrates and also the alignment directions in the vicinity of the pair of substrates holding therebetween the liquid crystal molecules shows the twisted positional relationship. By the way, for the twisted nematic mode, the several systems have been further proposed. As an example, there are an HFE (Hybrid Field Effect) mode, a TN-ECB (Twisted Nematic-Electrically Controlled Birefringence) mode, an SCTN (Self-Compensated Twisted Nematic) mode, an MTN (Mixed mode Twisted Nematic) mode, and the like.
The TN-ECB mode is described in an article of Japan Display ""89, p. 192 (1989), the SCTN mode is described in JP-A-10-090731, and the MTN mode is described in an article of Applied Physics Letters 68, p. 1455 (1996). In addition, the inclusive analysis of the overall reflection type twisted nematic mode is described in an article of Proceedings of SPIE 3635, p. 87 (1999) and an article of Proceedings of IDW ""99, p. 985 (1999).
By the way, of these modes, the HFE mode is the so-called normally black type display mode which becomes the black (dark) display when the voltage applied thereto is 0 Vrms, while becomes the white (light) display when a suitable voltage is applied thereto. Each of other three modes is the so-called normally white display mode which becomes the white (light) display when the applied voltage is 0 Vrms, while becomes the black (dark) display when a suitable voltage is applied thereto.
The typical system as the optical system employing these reflection type liquid crystal light valves is the optical system employing the polarizing beam splitter and it is described in JP-A-61-13885. In addition, as for other example, the optical system in which the main optical axis is made oblique with respect to the vertical direction of the liquid crystal light valve is described in JP-A-4-319910.
By the way, as for the technique in which the reflection type liquid crystal light valve and the retardation plate are combined with each other, the technique in which the polarizing beam splitter, the xc2xc wave plate and the reflection type liquid crystal light valve are combined with one another and these elements are arranged in such a way that the fast axis or the slow axis of the xc2xc wave plate intersects perpendicularly the flat surface containing the incident light axis and the reflected light axis of the polarizing beam splitter is disclosed in JP-A-2-250026 and U.S. Pat. No. 5,327,270. In addition, the technique in which the retardation of the retardation plate combined with the polarizing beam splitter is made equal to or larger than 0.25 (i.e., xc2xc wavelength) is disclosed in U.S. Pat. No. 5,576,854. Also, the description of the polarizing controller including the double refraction material layer is made in JP-A-1-7021.
The above-mentioned article of Proceedings of SPIE 3635 p. 87 (1999) reports that the conditions containing the MTN mode in the normally white reflection type twisted nematic mode is most excellent in the balance of the characteristics of the light utilization efficiency, the chromaticity characteristics, and the driving voltage (contrast ratio).
However, if the high speed response characteristics of the liquid crystal is regarded as important, the conditions containing the TN-ECB mode is more desirable. The importance of the high speed response characteristics of the liquid crystal molecules will hereinbelow be described by taking as an example the case where the liquid crystal display element is employed in the display device of the personal computer.
The image signals in the personal computer normally consist of the image signals corresponding to at least equal to or larger than 60 frames per second. That is, in the case where the image is displayed on the display device of the personal computer, in order that the display may surely follow the image signals, the liquid crystal needs to have the response time which is equal to or shorter than a time corresponding to one frame, i.e., equal to or shorter than {fraction (1/60)} seconds=16.6 milliseconds. If the response time of the liquid crystal is equal to or larger than a time corresponding to one frame, then the image which is different from that obtained on the basis of the image signals is displayed on the display device, and hence it is recognized on the residual image, which is expected not to essentially appear, at the back of the movement direction of the image. The phenomenon is particularly remarkable in the moving image, which reduces remarkably the quality of the image. Therefore, in order to realize the liquid crystal display device having the excellent image quality, it is necessary to select the display mode of the liquid crystal showing the high speed response.
Then, since it is in general known that the response time of the liquid crystal is in proportion to the square of the thickness of the liquid crystal, the high speed response can be expected as the thickness of the liquid crystal is thinner. In this respect, since the conditions containing the TN-ECB mode has the desired standardized retardation which is smaller than that in the conditions containing the MTN mode, even when the same liquid crystal material is employed, the thickness of the liquid crystal layer in the conditions containing the TN-ECB mode can be made narrower than that in the conditions containing the MTN mode. As a result, in the case of the conditions containing the TN-ECB mode, the shortening of the response time of the liquid crystal, i.e., the high speed operation of the liquid crystal response can be promoted as compared with the MTN mode.
Furthermore, the reflection type liquid crystal valve other than the transmission type liquid crystal light valves is employed, whereby the response time can be shortened. This will hereinbelow be described.
There is the retardation as the component contributing to the modulation of light in the light valve. The retardation is expressed by the product of the thickness d of the liquid crystal layer and the anisotropy xcex94n of refractive index of the liquid crystal.
While in the case of the transmission type liquid crystal light valve, the retardation is expressed by dxcex94n, in the case of the reflection type liquid crystal light valve, since the optical path is folded back by the reflection mirror, the incident light reciprocates through the liquid crystal layer. In other words, in the case of the reflection type liquid crystal light valve, the retardation contributing to the modulation of light becomes two times as large as that in the case of the transmission type liquid crystal light valve, i.e., becomes 2dxcex94n and hence though the liquid crystal layers have the same thickness, in the case of the reflection type liquid crystal light valve, the retardation which is two times as large as that in the case of the transmission type liquid crystal light valve can be obtained. That is, in the case of the reflection type liquid crystal light valve, even when the same liquid crystal material having the anisotropy of refractive index is employed, the thickness of the liquid crystal layer which is required to obtain the desired retardation can be made generally half that of the transmission type liquid crystal light valve. As a result, the greatly high speed response can be expected for the reflection type liquid crystal light valve.
Therefore, it is one of main objects of the present invention to provide a liquid crystal display device in which the high speed response is possible.
In addition, in the transmission type liquid crystal light valve, in order to prevent the generation of the flicker due to the slight overlap of the D.C. voltages, the driving is carried out in such a way that the applied voltages to the adjacent pixels are opposite in polarity to each other. Moreover, since the lateral electric field is generated in the region between the adjacent pixels in which the applied voltages thereto are opposite in polarity to each other, the alignment of the liquid crystal goes out of order in that region. This, for example, results in the reduction of the contrast ratio of the leakage of light.
For this reason, in the transmission type liquid crystal light valve, the light is shut off for the region in which the alignment of the liquid crystal goes out of order, thereby preventing the reduction of the contrast ratio. However, this measures leaves the problem that the aperture rate as the rate which the aperture occupies in one pixel is largely reduced.
On the other hand, since in the reflection type liquid crystal light valve, about all of the region of the pixels can be covered with the reflection pixel electrodes, the reflection type liquid crystal light valve has the great feature that it is possible to realize the aperture rate, which is close to 100% without limit. But, of course, it is necessary to avoid the disorder of the alignment of the liquid crystal due to the above-mentioned lateral electric field, which is generated in the region between the adjacent pixels, as much as possible. For this reason, in the reflection type liquid crystal light valve, the frame inverting drive in which the polarity of the applied voltages is inverted every frame is carried out so that the applied voltages to the adjacent pixels do not become opposite in polarity to each other. For all that, for example, the lateral electric field may generate in the boundary region between the adjacent pixels for the white image and the black image in some cases.
In order that the alignment of the liquid crystal may be prevented from going out of order due to the lateral electric field, it is necessary to adjust properly the so-called pretilt angle as the angle between each of a pair of substrates and the liquid crystal molecules.
In order that the alignment of the liquid crystal may be prevented from going out of order due to the lateral electric field generated in the region between the adjacent pixels in the homeotropic alignment, it is necessary to incline the angle of the liquid crystal molecules on the substrate interface by several degrees with respect to the direction perpendicular to each of the pair of substrates. However, if the liquid crystal molecules are inclined, then the retardation is generated. As a result, the contrast ratio is reduced. That is, the lateral electric field-resistance and the contrast ratio show the trade-off relationship. Therefore, it is necessary to obtain the most suitable relationship by taking these facts into consideration.
It is therefore another object of the present invention to provide a liquid crystal display device in which the high contrast ratio can be obtained.
In order to solve the above-mentioned problems associated with the prior art, according to one aspect of the present invention, there is provided a liquid crystal display element including: a reflection type liquid crystal light valve having a liquid crystal layer held between a transparent electrode and a reflection electrode, and a plurality of pixel circuits for driving the liquid crystal layer; and a retardation plate, wherein the retardation plate is arranged in such a way that an optical axis (a slow axis or a fast axis) of the retardation plate and the polarizing direction of the incident polarized light to the retardation plate are slightly shifted from each other.
In addition, there is provided the liquid crystal display element in which the retardation of the retardation plate therein is generally xc2xc of the wavelength of the incident light to the retardation plate.
An example suitable for the liquid crystal layer in the liquid crystal display element of the present invention is the liquid crystal having the twisted nematic alignment. In this connection, the twisted angle is set to the range of about 50 to about 90 degrees, and also the angle of the retardation plate is set to the value which is larger than 0 degree, but is equal to or smaller than 10 degrees, whereby the high contrast ratio can be realized.
Or, in the above-mentioned construction, the twisted angle is set to the range of about 50 to about 90 degrees, and also the angle of the retardation plate is set to the value which is smaller than 90 degrees, but is equal to or larger than 80 degrees, whereby the high contrast ratio can be realized.
Another example suitable for the liquid crystal layer in the liquid crystal display element of the present invention is the liquid crystal having the hemotropic alignment. When a first alignment direction of the liquid crystal is 45 degrees, the angle of the retardation plate is set to the value which is smaller than 0 degree, but is equal to or larger than xe2x88x9210 degrees, whereby the contrast ratio can be realized.
Or, in the above-mentioned construction, the angle of the retardation plate is set to the value which is larger than 90 degrees, but is equal to or smaller than 100 degrees, whereby the high contrast can be realized.
In addition, in order to solve the above-mentioned problems associated with the prior art, according to the present invention, there is provided a liquid crystal display device including: a liquid crystal display element having a reflection type liquid crystal light valve having a liquid crystal layer held between a transparent electrode and a reflection electrode, and a plurality of pixel circuits for driving the liquid crystal layer, and a retardation plate; a polarizing optical element; and a light source, wherein the retardation plate is arranged between the liquid crystal light valve and the polarizing optical element; the retardation of the retardation plate is generally xc2xc of a wavelength of incident light to the retardation plate; and wherein the absolute value of an angle between an optical axis of the retardation plate and a polarizing axis of the polarizing optical element is larger than at least 0 degree, but is equal to or smaller than 10 degrees.
Furthermore, there is provided the liquid crystal display device in which the liquid crystal element in the liquid crystal display device is any one of the above-mentioned liquid crystal elements.